This invention relates generally to spacecraft attitude control systems and more particularly to a system using reaction wheels and means for removing accumulated angular momentum from these wheels.
In some spacecraft systems, three-axis attitude control is achieved by means of control torques developed by three reaction wheels in response to sensor-derived information. The three reaction wheels are usually mounted in the spacecraft in such a manner that their axes are parallel to an orthogonal set of body axes about which attitude control is to be maintained.
For example, in an earth-pointing spacecraft, one of the wheel axes is parallel to the spacecraft pitch axis, which nominally has an angular velocity corresponding to the orbital rate, i.e., one revolution per orbit. The remaining two wheel axes are aligned with the spacecraft yaw and roll axes, respectively. As a result of control loop action, in the attitude control system, to overcome the effect of external disturbance torques acting on the spacecraft, angular momentum is accumulated by the reaction wheels. Due to the reaction wheel momentum storage limitations (i.e. speed), means must be provided to periodically remove such accumulated angular momentum by the application of appropriate external control torques to the spacecraft.